1. Field of the Invention
Embodiments of the invention are most generally related to the field of fluid (liquid) electromechanics and to the generation and application of frequency-addressable electromechanical forces and their controllable effects on working fluids. More particularly, embodiments of the invention are directed to both bistable and analog (continuous) microfluidic devices and working fluid actuation methods based on the generation and application of frequency-addressable electrowetting-on-dielectric (hereinafter “EWOD”)- and liquid dielectrophoresis (hereinafter “DEP”)-based forces.
2. Description of Related Art
The underlying theory and application of an electric field to control the motion of a dielectric media have been known for some time. More than 100 years ago, Pellat demonstrated that a non-uniform electric field could dramatically influence the hydrostatic equilibrium of a dielectric liquid. Pohl originally used the term “dielectrophoresis” (DEP) to describe the phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. At one time, the U.S. space program explored various ways to utilize the basic force mechanisms discovered by Pellat to manage and control propellants and other liquids in zero-gravity applications. Thereafter, the term “liquid dielectrophoresis” was used to refer to the behavior of liquids subjected to non-uniform electric fields analogous to the particle behavior described by Pohl. The interested reader is directed, for example, to H. A. Pohl, The Motion and Precipitation of Suspensoids in Divergent Electric Fields, J. Appl. Phys. 22(7), 869-871 (1951); Pohl, H. A., Some effects of nonuniform fields on dielectrics, J. Appl. Phys., 29(8), 1182-1188 (1958); Pohl, H. A., Dielectrophoresis behavior of neutral matter in nonuniform electric fields, Cambridge University Press. Cambridge (1978); and Jones, T. B., On the Relationship of Dielectrophoresis and Electrowetting, Langmuir 2002, 18, 4437-4443, and the references cited therein. More recently, for example, a method was described for control of a liquid by an electric field that utilized the non-uniform electric field created by dielectric-coated, planar electrodes patterned on insulating substrates to control and manipulate sessile droplets and small volumes of aqueous liquid. Exemplary schemes included dielectrophoretic (DEP) actuation and the electrowetting-on-dielectric (EWOD) effect. (See, e.g., Wang, K-L and Jones, T. B., Frequency-dependent bifurcation in electromechanical microfluidic structures, J. Micromech. Microeng., 14, 761-768 (2004) and the references cited therein, the subject matter of which is incorporated herein by reference in its entirety to the fullest allowable extent). EWOD involves modifying the wetting properties of a solid material via an applied voltage. It has become a widely used tool for manipulating tiny amounts of liquids on surfaces.
In addition to the space program applications mentioned above, DEP- and/or EWOD-based mechanisms have been employed in various other technologies including, but not limited to, display systems and digital microfluidics.
Display systems, for example, are omnipresent. Televisions, portable computers, cell phones and flash drive-based media players are but a few examples of electronic devices that incorporate display technology. The use of various devices in all kinds of environments places high demands on the quality of images produced by image display devices. Consumers demand high brightness and contrast, good resolution, wide angle viewing, light weight, fashionable and ergonomic styling, economic value, and other attributes.
The term ‘electronic paper’ represents a contemporaneous display technology, usually operating in the reflective mode. In the 1990s, electronic paper based on the electrophoretic motion of particles inside small capsules was demonstrated and later commercialized. One drawback of these systems is their slow response speed, which was controlled by the velocity of the particles.
Electrowetting has more recently been demonstrated as a basis of reflective/transmissive display applications for video and other data formats. In display applications, electrowetting can be used to manipulate colored, liquid-based compositions in enclosed membranes to facilitate pixel-based imaging. Improvements in response time over electrophoretic-based applications, as well as published accounts of brightness and color attributes have been reported in the literature.
Despite the attractive results that are being reported with electrowetting-based reflective/transmissive display technology, improvements are still being sought. For example, a continuous application of voltage is required to maintain an “ON” state, thus an image is maintained only as long as voltage is applied to the pixels that are to remain transparent. Electrowetting-based displays therefore are not truly bistable, do not have inherent storage capability, and require external circuitry for image maintenance and refreshment.
In the burgeoning field of microfluidics, improved methods and apparatus for manipulating, mixing, separating, delivering, analyzing, and otherwise actuating small liquid samples, that are more efficient, lighter, less expensive, disposable, and otherwise improved over current implementations, are constantly being sought. Similarly, the fields of photonics and fiber-optical communications continue to benefit from improvements in control mechanisms that are more reliable, more responsive, easier to implement, less expensive, and otherwise improved over existing technological solutions. Previously, structures required individually-addressable electrodes and changes in voltage to manipulate fluids from one region of the structure to another. These systems are burdensome and electrically complex. The inventors recognized the advantages and benefits of systems that reduced this complexity and the associated costs and other inefficiencies associated therewith.
The advantages and benefits provided by the teachings disclosed herein and the embodiments of the invention disclosed and claimed will become more apparent to persons skilled in the art in view of the following description and drawings.